HIGH-RESOLUTION STRATIGRAPHY, PALYNOFACIES AND SOURCE ROCK POTENTIAL OF THE AGUA DE MADEIROS FORMATION (LOWER JURASSIC), LUSITANIAN BASIN, PORTUGAL

In the Lusitanian Basin (central-western Portugal), the Lower Jurassic carbonate-dominated succession is thought to have significant source rock potential. One of the most important units is the Agua de Madeiros Formation (Upper Sinemurian lowermost Pliensbachian) which is composed of alternating organic-rich marls and limestones including black shale horizons. This paper is based on a study of this formation at its type locality at S. Pedro de Moel in western Portugal. Data includes Total Organic Carbon (TOC) measurements, palynofacies analyses and results of Rock-Eval pyrolysis presented within a high-resolution lithostratigraphic framework. TOC contents were measured in some 200 samples from the Agua de Madeiros Formation covering a stratigraphic interval of 58 m, and vary widely up to a maximum of about 22 wt %. Kerogen assemblages are dominated by marine amorphous organic matter with varying contributions by phytoclasts and palynomorphs. A majority of the 85 samples analyzed by Rock-Eval pyrolysis have S-2 values above 10 mg HClg rock, reaching a maximum of 78 mg HClg rock. These high S-2 values are correlative with maximum values of the Hydrogen Index which averages 355 mg HClg TOC (maximum of 637 mg HClg TOC). However in spite of these indicators of source-rock potential, the Agua de Madeiros Formation in the study area is thermally immature or very early mature, as indicated by T-max values below 437 degrees C and average vitrinite reflectance values of 0.43 % R-o.